In electrostatography an image comprising an electrostatic field pattern, usually of non-uniform strength, (also referred to as an electrostatic latent image) is formed on an insulative surface of an electrostatographic element by any of various methods. For example, the electrostatic latent image may be formed electrophotographically (i.e., by imagewise Photo-induced dissipation of the strength of portions of an electrostatic field of uniform strength previously formed on a surface of an electrophotographic element comprising a photoconductive layer and an electrically conductive substrate), or it may be formed by dielectric recording (i.e., by direct electrical formation of an electrostatic field pattern on a surface of a dielectric material). Typically, the electrostatic latent image is then developed into a toner image by contacting the latent image with an electrostatographic developer. If desired, the latent image can be transferred to another surface before development.
One well-known type of electrostatographic developer comprises a dry mixture of toner particles and carrier particles. Developers of this type are commonly employed in well-known electrostatographic development processes such as cascade development and magnetic brush development. The particles in such developers are formulated such that the toner particles and carrier particles occupy different positions in the triboelectric continuum, so that when they contact each other during mixing to form the developer, they become triboelectrically charged, with the toner particles acquiring a charge of one polarity and the carrier particles acquiring a charge of the opposite polarity. These opposite charges attract each other such that the toner particles cling to the surfaces of the carrier particles. When the developer is brought into contact with the latent electrostatic image, the electrostatic forces of the latent image (sometimes in combination with an additional applied field) attract the toner particles, and the toner particles are pulled away from the carrier particles and become electrostatically attached imagewise to the latent image-bearing surface. The resultant toner image can then be fixed in place on the surface by application of heat or other known methods (depending upon the nature of the surface and of the toner image) or can be transferred to another surface, to which it then can be similarly fixed.
A number of requirements are implicit in such development schemes. Namely, the electrostatic attraction between the toner and carrier particles must be strong enough to keep the toner particles held to the surfaces of the carrier particles while the developer is being transported to and brought into contact with the latent image, but when that contact occurs, the electrostatic attraction between the toner particles and the latent image must be even stronger, so that the toner particles are thereby pulled away from the carrier particles and deposited on the latent image-bearing surface. In order to meet these requirements for proper development, the level of electrostatic charge on the toner particles should be maintained within an adequate range.
The toner particles in dry developers often contain material referred to as a charge agent or charge-control agent, which helps to establish and maintain toner charge within an acceptable range. Many types of charge-control agents have been used and are described in the published patent literature.
One type of material that has been used in the prior art as a charge-control agent is a .beta.-diketone metal complex. For example, Japanese Patent Application No. 80/156325, filed Nov. 6, 1980 (published unexamined Application [Kokai] No. 57/079964, laid-open May 19, 1982 describes an electrostatographic developing process which employs a positive charging single component developer containing a .beta.-diketone metal complex that can be represented by the structure: ##STR2## where R.sub.1 and R.sub.2 are alkyl, and M.sup.n is an n-valent metal.
Japanese Patent Application No. 85/53348, filed Mar. 19, 1985 (published unexamined application [Kokai] 61/212852, laid-open Sep. 20, 1986) describes an electrostatographic toner comprising as a charge-control agent a metal complex of a .beta.-diketone represented by the structure: ##STR3## where R.sub.1 and R.sub.2 are hydrogen, halogen, alkyl, aralkyl, alkoxy, aryloxy, aralkyloxy, alkylthio, arylthio, aralkylthio or nitro in which R.sub.1 and R.sub.2 may be same or different and R.sub.1 and R.sub.2 together may form a substituted or unsubstituted ring; R.sub.3 is alkyl, aryl or aralkyl; the alkyl or aryl in each of R.sub.1, R.sub.2 and R.sub.3 may be substituted; M is metal; X is a neutrally coordinated molecule; m is an integer of 1 to 3; and n is an integer of 0, 2 or 4.
Unfortunately, many of the prior art known charge-control agents, including the .beta.-diketone metal complexes described in the aforementioned Japanese patent applications exhibit significant drawbacks. For example, such charge agents often fail to provide sufficient initial charge of a desired negative or positive polarity or to maintain such charge for adequate development throughout the development process. In addition, certain charge control agents are highly colored and often impart an undesirable color to the toner which is objectionable in colored developers.
Also, poor dispersibility of some charge agents in some of the known toner binder materials can exacerbate some of the problems mentioned hereinbefore. Non-uniform dispersion of charge agent means that higher concentrations or agglomerations of charge agent will exist in some portions of the toner binder mix, compared to others. In typical melt-blending processes, the toner mixture is cooled and ground down to desired particle size after melt-blending. Agglomerations of charge agent provide sites in the mixture where fracture is more likely to occur during grinding. The new surfaces created by such fracture will have a higher concentration of charge agent than will internal sites. Thus, the final toner particles will have a higher surface concentration of charge agent than internal concentration. It should be readily appreciated that if a charge agent tends to adversely interact with the environment, higher surface concentrations of charge agent on the toner particles will lead to a greater degree of such interaction, thus exacerbating such problems as high environmental sensitivity.
It would, therefore, be desirable to provide new electrostatographic toners and developers that could perform the charge-controlling function well, while avoiding or minimizing the drawbacks noted hereinbefore. The objective of this invention is to provide such electrostatographic toners and developers.